1. Field of the Invention
The invention relates to an apparatus and a method for dry etching used in fabrication process of a semiconductor device, and more particularly to an apparatus and a method for dry etching a silicon nitride film deposited on a semiconductor substrate.
2. Description of the Related Art
A process of fabricating a semiconductor device generally includes a step of electrically isolating a plurality of semiconductor elements from one another. To this end, there is usually used a method of dry etching a silicon nitride film deposited on a semiconductor substrate.
FIGS. 1A to 1C are partial cross-sectional views of a semiconductor device, illustrating respective steps of a method of dry etching. FIG. 1A illustrates an object to be dry etched. As illustrated in FIG. 1A, the object is comprised of a semiconductor substrate 40, a silicon dioxide film 41 formed on the semiconductor substrate 40 by thermally oxidizing the semiconductor substrate 40, a silicon nitride film 42 formed on the silicon dioxide film 41, and a photoresist 43 formed on the silicon nitride film 42 and patterned by means of photolithography.
As illustrated in FIG. 1B, the silicon nitride film 42 is dry etched by means of a parallel plate type reactive ion etching (RIE) apparatus. Then, the patterned photoresist 43 is removed, followed by thermal annealing. Thereafter, the silicon nitride film 42 and the silicon dioxide film 41 are removed. Thus, there are formed element isolation regions 44 at a surface of the semiconductor substrate 40, as illustrated in FIG. 1C.
There have been employed various etching gases for dry etching the silicon nitride film 42. For instance, followings are used as an etching gas: a gas including fluorocarbon gas, such as CHF.sub.3 /O.sub.2 and CF.sub.4 /CHF.sub.3 ; SF.sub.6 /CHF.sub.3 suggested in Japanese Unexamined Patent Publication No. 5-251399; CF.sub.4 /H.sub.2 suggested in Japanese Unexamined Patent Publication No. 56-122129; CF.sub.4 IN.sub.2 suggested in Japanese Unexamined Patent Publication No. 1-214025; and CF.sub.4 /O.sub.2 suggested in Japanese Patent Publication No. 63-41987.
The above-mentioned conventional apparatus and method for dry etching has problems as follows.
The first one is as follows. When CHF.sub.3 /O.sub.2, CF.sub.4 /CHF.sub.3 or SF.sub.6 /CHF.sub.3 is used as an etching gas, etching is not balanced with deposit in a semiconductor substrate, since CHF.sub.3 gas has a characteristic of deposition. As a result, it would not be possible to have uniformity in etching. According to the results of the experiments the inventor has conducted, it was not possible to have uniformity smaller than .+-.10% (3 mm edge exclusion) in a 8-inch silicon substrate in the case that a gas including CHF.sub.3 was used as an etching gas.
The second problem is that if CF.sub.4 /N.sub.2 or CF.sub.4 /O.sub.2 is used for dry etching a silicon nitride film, selectivity between the silicon nitride film and photoresist is reduced because of addition of N.sub.2 or O.sub.2, which makes it difficult to have desired dimensions. Namely, the use of CF.sub.4 /N.sub.2 or CF.sub.4 /O.sub.2 causes difficulty in controlling a dimension in a silicon nitride film to be etched.
The third problem is as follows. A gas supplying plate in an etching chamber in a conventional dry etching apparatus, through which an etching gas is introduced into the etching chamber, is in general made of aluminum or alumite. Hence, there is a problem that when the gas supplying plate is exposed to plasma, aluminum reacts with fluorine contained in an etching gas to thereby unpreferably generate particles. According to the results of the experiments the inventor has conducted, in the case that a gas supplying plate made of aluminum or alumite was used in dry etching with SF.sub.6 /CHF.sub.3 family gas used as an etching gas, particles had suddenly increased when the total charge time had reached about 40 hours.
The fourth problem is as follows. There are two ways in dry etching a silicon nitride film. One of them is so-called LOCOS process where a silicon dioxide film lying under a silicon nitride film is kept unetched. The other is so-called RECESS process where a silicon nitride film, a silicon dioxide film lying under the silicon nitride film, and a semiconductor substrate disposed under the silicon dioxide film are all etched by 50-70 nm. In the conventional dry etching apparatus and method, it is not possible to commonly use a gas in these two processes.
For instance, if CHF.sub.3 /O.sub.2 or CF.sub.4 /H.sub.2 is used as an etching gas, an etching rate for silicon is not obtained in the RECESS process, resulting in lack of mass-productivity.
If CF.sub.4 /CHF.sub.3 is used as an etching gas in the LOCOS process, only poor uniformity in etching it obtained, as mentioned earlier. The poor etching uniformity makes is difficult to detect an interface between a silicon nitride film and a silicon dioxide film, at which etching should be ceased, and as a result, it would be quite difficult to keep the silicon dioxide film unetched.
If SF.sub.6 /CHF.sub.3 is used as an etching gas in the RECESS process, an etching rate for a semiconductor substrate or a silicon substrate is quite high, 20 specifically, hundreds of nanometers per a second. Hence, when a semiconductor substrate is to be etched only by tens of nanometers, it would be difficult to control etching with the result of poor reproducibility.
If CF4/N2 or CF4/O2 is used as an etching gas, no problems would be posed about mass-productivity and reproducibility in both LOCOS and RECESS processes, however, a problem about difficulty in controlling dimensions remains unsolved, as mentioned earlier.
There has been suggested a poly-etcher plasma source in TEL NEWS/SPE Vol. 43, October 1996, pp. 13-14. As illustrated in FIG. 3, the suggested plasma source includes a process chamber, top and bottom matchers, RF generators electrically connected to the top and bottom matchers, respectively, and an IEM (Ion Energy Modulation) controller electrically connected to the RF generators and detectors. It is said that middle density plasma with high stability can be obtained.